Re: Speed up Clog Access by increasing CLOG buffers

On Mon, Mar 20, 2017 at 8:27 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Fri, Mar 17, 2017 at 2:30 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>> I was wondering about doing an explicit test: if the XID being
>>> committed matches the one in the PGPROC, and nsubxids matches, and the
>>> actual list of XIDs matches, then apply the optimization. That could
>>> replace the logic that you've proposed to exclude non-commit cases,
>>> gxact cases, etc. and it seems fundamentally safer. But it might be a
>>> more expensive test, too, so I'm not sure.
>>
>> I think if the number of subxids is very small let us say under 5 or
>> so, then such a check might not matter, but otherwise it could be
>> expensive.
>
> We could find out by testing it. We could also restrict the
> optimization to cases with just a few subxids, because if you've got a
> large number of subxids this optimization probably isn't buying much
> anyway.
>

Yes, and I have modified the patch to compare xids and subxids for
group update. In the initial short tests (with few client counts), it
seems like till 3 savepoints we can win and 10 savepoints onwards
there is some regression or at the very least there doesn't appear to
be any benefit. We need more tests to identify what is the safe
number, but I thought it is better to share the patch to see if we
agree on the changes because if not, then the whole testing needs to
be repeated. Let me know what do you think about attached?

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com